1. Field of the Invention
The present invention relates to an apparatus for laser ranging, and more particularly to a laser range finder for use on a satellite for inter-satellite ranging.
2. Description of the Related Art
For accurate inter-satellite time synchronization, it is necessary to accurately measure the inter-satellite range, which runs up to 70,000 km.
One apparatus for accurately measuring long ranges is a laser range finder using short-pulse laser (see, for example, Japanese laid-open patent publication No. 10-239600). The conventional laser range finder emits a short-pulse laser beam toward a target and detects the laser beam reflected from the target to measure the distance up to the target. This range-finding process will be hereafter referred to as a reflected beam reception process.
If a moving object such as a satellite is a target, then the laser range finder directs the laser beam toward the target while tracking the moving target (the process will hereinafter referred to as pointing). Tracking the target requires positional information of the target. Usually, since the positions of satellites are managed on the ground, when a satellite measures the distance up to another satellite, the positional information can be given to the ranging satellite from the ground.
However, the conventional laser range finder suffers the following problems:
For ranging according to the conventional reflected beam reception process, since the laser beam needs to travel to the target and then back, a laser system having a large output capability and a large size is required. For example, for measuring the distance up to a satellite that is about 40,000 km high from the ground, then a laser output capability of about 1 joule is required even if the satellite has a laser reflecting mirror such as a corner cube reflector, and the laser having a large size of 3 m×1.5 m×1.5 m has to be used. Therefore, the conventional reflected beam reception process is not suitable for use on satellites for inter-satellite ranging.
The positional information that is sent from the ground to a satellite for tracking a moving satellite to point the laser beam suffers a large positional error and is not in real-time. Therefore, the positional information from the ground is not sufficient for use on the laser range finder on the satellite for tracking the target.
Another problem is that laser range finders on satellites have their laser oscillators tending to cause a misalignment due to harsh environmental changes in space, and hence have a reduced laser output capability.
Because of the various factors described above, it is difficult for the conventional laser range finder to measure the inter-satellite range sufficiently accurately in real-time.